Perhaps if you give us some mor information about your actual intent, we could provide more helpful information.

Now that new information has been provided in the question, I would suggest to use the algorithmicx package to define the new block; while the algorithmic package doesn’t allow you to easily modify predefined structures, or to create new ones, the algorithmicx package gives you full control over the definitions. Using algcompatible, you can have full compatibility with the syntax of the algorithmic package; a little example of the definition of the new block: